Fog reduction in silver halide emulsions with 3-mercaptobenzoic acid



United States Patent 3,226 231 FOG REDUCTIQN 1N SILVER HALIDE EMULSIONS WITH 3-MERCAPTOBENZOIC ACID Fritz Dersch and Charles A. Clark, Binghamton, N.Y., assignor to General Aniline & Film Corporation, New York, N.Y., a corporation of Delaware No Drawing. Filed May 16, 1963, Ser. No. 281,053 Claims. (Cl. 96-61) This invention relates to photographic materials and to methods for preparing them. More particularly it relates to light-sensitive silver halide emulsions containing compounds which serve as stabilizing and antifogging agents.

It is known in photographic art that light-sensitive gelatino silver halide emulsions have a tendency to fogfor example, upon prolonged ripening of the emulsions, prolonged storage of film or paper bearing the emulsion especially at elevated temperature and humidity-and upon prolonged development. The tendency to fog is especially pronounced in rapid development processes carried out at elevated temperatures.

A great number of chemical compounds have been described in the prior art as effective to reduce fog in lightsensitive silver halide emulsions, thus increasing their stability. However, most of these compounds are inadequate when the film or paper bearing the emulsion is subjected to rapid development at elevated temperatures. Moreover, many of the stabilizers and .an'tifogging agents of the prior art have the disadvantage of causing decreased sensitivity, resulting in loss of speed and/ or contrast of the emulsion when developed.

It is accordingly a primary object of this invention to produce light-sensitive silver halide emulsion containing stabilizing and fog inhibiting agents which do not appreciably lower the sensitivity of the emulsion.

A further object of the invention is to provide light-sensitive silver halide emulsions which do not develop excessive fog during rapid development at elevated temperatures. We have discovered that the above objects can be accomplished by adding to a light-sensitive silver halide emulsion 3-mercaptobenzoic acid. While it was known before (eg. in US. Patent 2,377,375) that the corresponding ortho isomer (thiosalicylic acid) is an antifoggant, we have found that the meta compound is far superior to the ortho compound, especially in those cases where hightemperature rapid-processing is applied.

References disclosing preparation of 3-mercaptobenzoic acid are given in Beilstein, Handbuch der Organischen Chemie (4th Edition) Vol. X, p. 148 (and second supplement. 3-mercaptobenzoic acid can be prepared by adding 35 g. of zinc dust in small portions over a period of six hours to a solution of 20 g. of chlorosulfonylbenzoic acid in 150 ml. of ethanol and 100 ml. of concentrated hydrochloric acid while agitating the mixture, filtering out undissolved zinc and zinc salts, mixing the filtrate with solid ferric chloride to precipitate diphenyldisulfide-3,3'- dicarboxy acid in crystalline form, recovering the latter product by filtering, washing successively with water and acetone, and finally re-crystallizing from glacial acetic acid. The cream colored needles (M.P. 246 C.), insoluble in water and poorly soluble in most organic solvents, represent a yield of about 67% of theory. The disulfide is converted to B-mercaptobenzoic acid by slurrying in aqueous 2 N caustic soda for 2 hours at 18 C., acidifying the resulting solution with excess sulfuric acid, filtering and recrystallizing the filter cake from aqueous alcohol. The product forms colorless needles having a melting point of 1467 C.

The antifogging agent of the invention may be added to the emulsion at any stage during its process of production. Thus, it may be added as a ripening final or as a coating final. When added as a ripening final, it is added during the ripening or sensitivity-increasing stage of the emulsion-making process. Such addition may be made before, during or after the addition of a soluble silver salt to the soluble halide in the presence of a suitable colloid, such as gelatin, polyvinyl alcohol, solubilized casein or albumin. When added as a coating final, the antifogging agent of the invention is added to the emulsion just prior to coating it on a suitable support such as glass, paper, or film at a time when the emulsion has nearly attained its maximum sensitivity.

In some instances it is advantageous to apply the antifogging and stabilizing compound of the invention in a separate layer such as an undercoating or in an anti-abrasion gelatin surface layer. In some cases it is desirable to incorporate the compound in one or all processing baths for the light-sensitive material such as the developer solution, or the preand postbaths.

When used as a ripening final, the antifoggant of this invention is preferably added to the emulsion in an amount ranging from 0.02 to 5 mg. per 0.6 gram-mol of silver halide, and when used as a coating final it is preferably added in an amount ranging from 1 to 20 mg. per 0.6 gram-mol of silver halide. In many emulsions, it is preferable to apply the antifoggant for best effectiveness in a gelatin surface coating. The optimum amount depends primarily on the type of emulsion which is used and should be determined individually in each case.

The stabilizer and antifoggant of the invention may also be used in combination with other known antifoggants and stabilizers, and also in combination with sensitizers as sulfur-, reduction-, metaland noble metal sensitizers as well as in combination with speed-increasing agents and accelerators such as polyoxyalkylenes and their derivatives.

The antifoggant and stabilizer of the invention may be used with various types of photographic emulsions such as non-sensitized, orthochromatic, panchromatic and X-ray emulsions, paper emulsions and color emulsions.

The following examples, wherein parts and percentages are by weight unless otherwise indicated, are given as an illustration of the manner in which the antifoggant of this invention can be used. It is to be understood however that the examples are given by way of illustration and not by way of limitation.

Example I A silver halide emulsion in gelatin containing 2% silver iodide and 98% silver bromide was prepared in a conventional manner and brought up to its maximum light-sensitivity. It was then readied for coating, finals were added such as sensitizing dyes and hardening agents and a 0.1% solution of 3-mercaptobenz0ic acid was added in varying amounts to samples of the emulsion as an antifoggant and stabilizer. The emulsion samples contained about 0.6 gram-mol of silver halide. The thus prepared emulsion samples were coated on a cellulose ester film base and dried. Samples of the film thus coated were then exposed 3 in a type IB Sensitometer and developed in a developer having the following composition:

G. Metol 1.5 Sodium sulfite (anhydrous) 45 Sodium. bisulfite 1 Hydroquinone 3 Sodium carbonate (monohydrate) 6 Potassium bromide 0.8

Water to make one liter.

The developed samples were short-stopped, fixed, washed and dried. The results were as follows:

Quantity of autifoggant Relative Fog at 12 Oven fog at used, mg. Speed minute de- 3 minute velopment development Example II Example III A gelatin-silver halide emulsion containing 2% silver iodide and 98% silver bromide was coated on a film base in a manner known to the art. After coating, an aqueous gelatin solution containing 20 g. of gelatin and 80 mg. of 3-mercaptobenzoic acid in one liter of Water was coated on the face of the emulsion as an anti-abrasion layer. After drying, film samples were exposed and processed as described in Example I. The samples exhibited a relative speed of 100 and a fog of 0.12 compared with a control coating of the above emulsion having an antiabrasion layer similar to that described above, but lacking the antifoggant. The control sample had a speed of 100 and a fog of 0.18.

Example IV The process of Example III was repeated except that development was carried out at a temperature of 105 F. In this case, the control sample containing no antifoggant developed to a fog of 0.67. A sample, prepared with 50 ml. of 0.1% Z-mercaptobenzoic acid (thiosalicylic acid) and 20 g. of gelatin per liter of surface coating solution to produce the anti-abrasion layer, developed to a fog of 0.56. In the case of a sample prepared with a solution containing 50 ml. of a 0.1% 3-mercaptobenzoic acid solution and 20 g. of gelatin per liter in order to prepare the anti-abrasion layer, the fog developed was only 0.17 and no speed loss was encountered.

Similar results are obtained when development was carried out at temperatures ranging from F. to F.

The invention is not limited to the detailed description contained herein, but includes modifications within the scope of the appended claims.

We claim:

1. A light-sensitive silver halide emulsion containing as an antifogging and stabilizing agent 3-mercaptobenzoic acid.

2. A light-sensitive silver halide emulsion as defined in claim 1 wherein the antifogging and stabilizing compound is present in the emulsion in a ratio of 0.02 to 20 mg. per 0.6 gram-mol of silver halide.

3. A light-sensitive silver halide emulsion as defined in claim 1 containing an optical sensitizer.

4. A light-sensitive silver halide emulsion as defined in claim 1 containing an accelerator.

5. A light-sensitive photographic element comprising a base and a coating of a gelatin-silver halide emulsion thereon, said emulsion containing as an antifogging and stabilizing agent 3-mercaptobenzoic acid.

6. A light-sensitive structure comprising, a base, a layer of light-sensitive silver halide emulsion thereon and an adjacent layer separate from the first mentioned layer containing 3-mercaptobenzoic acid.

7. A process of forming a photographic emulsion having a reduced tendency to fogging which comprises forming the emulsion, ripening the emulsion and, during said ripening adding thereto as an antifogging and stabilizing agent, 3-mercaptobenzoic acid.

8. A process of forming a light-sensitive photographic element having a reduced tendency to fog which comprises forming an emulsion, ripening said emulsion, coating said emulsion on a base and adding to the emulsion just prior to the coating thereof on the base S-mercaptobenzoic acid.

9. A process of minimizing and preventing fog in a light-sensitive silver halide material comprising a base having a light-sensitive emulsion thereon, which comprises exposing said emulsion to light and developing the same in the presence of 3-mercaptobenzoic acid as an antifoggant.

10. The process as defined in claim 9 wherein the development takes place at a temperature ranging from 95 to 120 F.

OTHER REFERENCES Beilstein: Handbuch der Organischen Chemie, 4th Ed., Vol. X, page 148 (1927).

NORMAN G. TORCHIN, Primary Examiner. 

1. A LIGHT-SENSITIVE SILVER HALIDE EMULSION CONTAINING AS AN ANTIFOGGING AND STABLIZING AGENT 3-MERCAPTOBENZOIC ACID. 